Conditional Requests

1. Introduction

Conditional requests are a mechanism in the HTTP protocol that allows a client to make a request for a resource only if certain conditions are met. This is primarily used for caching and performance optimization, reducing unnecessary data transfer and improving load times.

2. Key Concepts

• HTTP Headers: Conditional requests use specific HTTP headers like If-Modified-Since and If-None-Match.
• ETags: Entity tags (ETags) are unique identifiers assigned to specific versions of a resource, allowing clients to check if the resource has changed.
• Cache-Control: HTTP headers that dictate caching behavior can optimize conditional requests.

3. How It Works

A conditional request is made by including specific headers in the HTTP request. Here's a step-by-step breakdown:

1. Client makes an initial request to the server and receives a resource along with an ETag or a Last-Modified timestamp.
2. For subsequent requests, the client sends an If-None-Match or If-Modified-Since header with the stored ETag or timestamp.
3. The server checks the condition:
   • If the resource has not changed, the server responds with a 304 Not Modified status.
   • If the resource has changed, the server responds with the updated resource and a 200 OK status.

4. Flowchart

graph TD;
    A[Client Makes Initial Request] --> B[Server Sends Resource with ETag];
    B --> C[Client Stores ETag];
    C --> D[Client Makes Conditional Request];
    D --> E{Resource Changed?};
    E -->|No| F[Server Sends 304 Not Modified];
    E -->|Yes| G[Server Sends Updated Resource with 200 OK];
            

5. Best Practices

• Always use ETags for versioning resources.
• Implement cache validation to reduce server load.
• Use appropriate cache control headers to optimize caching behavior.

6. FAQ